I have a 1f78 model thermostat. Everything appears to work ,except when button put to heat, unit clicks off. I replaced batteries. Pilot lite/furnace works fine in other zones.

You say other zones work fine, which makes me think you have more thermostats for those zones? Easiest way to trouble shoot is to take one of the other stats and put in in place of the one that is giving you fits. If it works just fine, you need a new stat.